The SPI module is enabled by setting SPE of SPCR (As long as its Power Reduction flag PRSPI† is not set.)
Data is shifted from Master to Slave on the Master Out - Slave In (MOSI) line, and from Slave on Master In - Slave Out(MISO).

On the master, the program clears SS for the specific slave device.
Writing to SPDR initiates the transfer. When compete, End of Transmission Flag( SPIF) is set and an interrupt may be triggered.
The master the sends another byte or sets SS

Arduino has has pins for analog input,
for soures ranging from 0 to 5 volts, ( Aref is connected to VCC) .
These are convert it to a number between 0 (0 volts) and 1023 (5 volts) (10 bits of resolution).